x86/PCI: Allow DMA ops specific to a PCI domain

The Intel Volume Management Device (VMD) is a PCIe endpoint that acts as a
host bridge to another PCI domain.  When devices below the VMD perform DMA,
the VMD replaces their DMA source IDs with its own source ID.  Therefore,
those devices require special DMA ops.

Add interfaces to allow the VMD driver to set up dma_ops for the devices
below it.

[bhelgaas: remove "extern", add "static", changelog]
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>